The present invention relates to a method of forming a shaped hole in a component. In particular the present invention relates to the laser drilling of a cooling hole in a component of a gas turbine engine.
Components in a gas turbine engine are often cooled to survive the high temperatures found therein. One method used is thin film cooling by a supply to air, used as a cooling fluid, is provided to a passage within the component, exiting via cooling holes to form a thin film over the external surface of the component. In addition to removing heat from the component by conduction, the thin film of cooling air serves to prevent hot gas within the gas turbine engine impinging upon the component.
The cooling air used for thin film cooling must be supplied at a pressure greater than the gas path pressure in which the component is operating. This requires work to be carried out on the cooling air, representing a loss of useful power from the engine. In order to minimise the work required and hence maximise the efficiency of the engine, the cooling holes are shaped to optimise the use of cooling air. The shape most commonly used is a 2D diverging hole exiting at low incidence to the surface of the component to be cooled. This xe2x80x9cfanxe2x80x9d shaped hole allows the cooling film to form more readily than would a conventional constant-bore hole.
At present, such cooling holes are manufactured using EDM spark erosion. This places the component and an electrode in a dielectric such as kerosene or de-ionised water. When a voltage is applied across a small gap between the electrode and the component, material is spark eroded from both. By advancing the electrode into the component a hole is formed. Furthermore, by moving the feed axis of the electrode relative to the component, different shapes can be produced.
However, the EDM process has weaknesses. Electrodes tend to bend or skid when used at very shallow angles to the surface. When combined with other problems, such as arcing or damaged electrodes, the repeatability of the process can be poor leading to high levels of scrap. Other problems include lack of flexibility, due to the need for bespoke tooling for each component configuration and a slow process time.
Rolls-Royce""s European Patent Application No 89310322.6 describes a laser drilling apparatus for drilling shaped holes in a workpiece. However the holes produced by the apparatus are novel in geometry. The present invention provides the ability to laser drill cooling holes that closely approximate xe2x80x9cconventionalxe2x80x9d cooling holes. This is advantageous as the technology can be used to manufacture existing engine components without necessitating redesign.
According to the present invention there is provided a method of forming a through hole in a component between a first surface and a second surface thereof, the through hole comprising a cavity formed in the first surface and a communicating passage between the cavity and the second surface comprising the steps of:
laser percussion drilling a plurality of blind holes in the first surface to form the cavity,
then forming the communicating passage between the cavity and the second surface.